The standard for optical transport of telecommunications in North America is Synchronous Optical Network or SONET and its European variant is Synchronous Digital Hierarchy or SDH. The SONET and SDH standards specify various protection schemes such as line protection and path protection. Conventionally, a line comprises one or more SONET sections or spans between SONET network elements. A path is a logical connection between a point where a Synchronous Transport Signal (STS) or a Virtual Tributary (VT) is multiplexed into the transport network and the point where the signal is demultiplexed.
Line and path protection schemes depend upon the various transport network architectures in which the schemes are operating, such as linear networks and ring networks (virtual line-switched ring or VLSR, unidirectional path-switched ring or UPSR, two- and four-fiber bi-directional line switched rings or BLSR). Linear protection schemes include 1+1, 1:1, and 1:N. For 1+1 and 1:1 protection schemes, one protection path serves to protect one working path or one section protects another section. For the 1+1 protection scheme, for example, the head-end network element permanently bridges the head-end signal to both working and protection equipment to transmit the identical payload to respective working and protection equipment of the tail-end network element. The tail-end network element continuously monitors both the working and protection signals for failures and automatically selects the protection signal in the event of an optical fiber or node failure.
A 1:N protection scheme enables a single optical protection path or section to protect any one of N working paths or sections. Criteria for detecting path failure are based on parameters such as an alarm indication signal (AIS), a loss of pointer (LOP), signal degrade, and excessive bit-interleaved parity errors. Protection signaling between the head-end network element and the tail-end network element occurs over the SONET Automatic Protection Switching (APS) channel, using bytes K1 and K2 in the line overhead.
As an example of a ring protection scheme, four-fiber BLSR technology uses four fibers between adjacent network elements, two of the four fibers for carrying working traffic and the other two fibers for carrying protection traffic. The working traffic travels in opposite directions on its two fibers, while the protection traffic travels in opposite directions on its two fibers. The protection fibers are available to transport any traffic needing bandwidth in the event of a failure in the network. In effect, this bandwidth is shared among all the circuits in the ring.
Each protection scheme has associated requirements. One requirement is the time within which a network element is to identify a fault, and another is the time within which to take corrective action. For example, for an STS-managed BLSR network architecture with no extra traffic, the network element needs to detect a fiber cut in less than 10 ms and then switch the traffic to a redundant fiber. This switch needs to occur throughout the ring in less than 50 ms. Other network architectures have different timing requirements.
To satisfy these timing requirements during a protection switch operation, a network element traditionally embedded protection signaling in the data stream (i.e., referred to as in-band signaling), using unused or borrowed bytes from the protocol. The success of such in-band signaling relied upon the functional compatibility among those devices that inserted and extracted the protection information into and from the data stream. Compatibility between in-band signaling mechanisms, however, is generally absent between devices produced by different equipment vendors. There is a need, therefore, for a protection signaling mechanism and method capable of achieving the low latency and high bandwidth requirements for supporting line and path protection in a communications network.